Electronically switched DC motors are used in many control and regulation applications. Such motors are also used in peripherals, such as mass memory drive systems like hard disks, floppy disks, optical disks and CD-ROM drives, as well as in tape drives and the like, all generally requiring ever increasing speeds. Although these systems may be easily designed for high speed, the inductive character of the electrical load represented by the phase windings of the motor, the inertial characteristics of the rotor and of the rotating mass, pose limitations and create technological problems for the realization of motors capable of being correctly controlled at ever increasing speeds.
Despite the fact that electronically switched systems for brushless DC motors may be designed for a higher number of phase windings, it is quite common for a brushless motor to have three phase windings connected in a star configuration and defining six different switching phases and P number of poles. In this case, there will be a number 3*P of equilibrium points in a complete revolution of the rotor.
In the present description, each excitation phase will be indicated according to a standard notation by two capital letters. The first capital letter (for example, A, B or C) defines the winding through which the current conventionally flows from the respective supply terminal towards the star center (CT). The second capital letter, preceded by the sign (.backslash.), designates the winding through which the current, conventionally coming from the star center (CT), flows toward the supply terminal of the relative winding.
These brushless motors are commonly driven by an integrated circuit whose output stage is represented by a polyphase full-wave bridge circuit, which in the case of a three-phase motor may employ six bipolar (BJT) or power field effect transistors (MOS). Commonly, the motor current is linearly controlled through a transconductance loop or in a PWM mode as shown in the scheme of FIG. 1. During a certain switching phase of the motor, the "sourcing" power transistor is forced into full conduction (that is to saturation in case of an MOS device) whereas the "sinking" power transistor operates as a transconductance element.
The techniques of electronically switching DC motors permit control of the rotation according to different modes. Some of these control modes are described in the European patent applications No. 96830440.2, filed on Aug. 1, 1996, No. 96830295.0, filed on May 22, 1996, No. 96830190.3, filed on Apr. 4, 1996 and No. 96830180.4 filed on Mar. 29, 1996, assigned to the present assignee.
During each switching phase, regardless of the fact that a voltage or current control mode is implemented, the driving of the motor windings may involve either a constant or varying current. The case of a three-winding DC motor, driven with a constant current during each phase, implies the excitation of two windings during each switching phase, while a third winding remains unexcited. This driving scheme whereby at any instant the current flows only in two windings out of three is often referred to as "bipolar".
By referring to A, B and C the three winding terminals and by calling AB.backslash. the supply between phase A and phase B with the A potential greater than the B potential, the required sequence for obtaining a rotation is: EQU AB.backslash.-AC.backslash.-BC.backslash.-BA.backslash.-CA.backslash.-CB.ba ckslash.
When the motor must rotate in the opposite direction it is sufficient to invert the direction of the above sequence.
For an efficient driving of the motor, the phase switching instant must be synchronized with the instantaneous rotor's position. This type of driving is known as "bipolar" because two phase windings are exited at each instant, and in case of a motor connected in a star configuration, permits controlling the current in the winding by using, as mentioned, a single sensing resistor in electrical series with the common source of the three half-bridges. Indeed, the phases (windings) that at any instant are crossed by the driving current are connected in series. Therefore, only a single control of the current that crosses both windings and also the sensing resistor is required.
The voltage produced by the current on the terminals of the sensing resistor is used as a feedback by the current control circuit. The current control circuit may be of the linear type, controlling for instance the saturation voltage transistors of the lower MOS of the half-bridge, or of the PWM type, switching off for a certain period, for example, the upper MOS transistors or the half-bridges.
Differently from a motor connected in a star configuration, the motor with independent windings whose driving scheme is shown in FIG. 2, does not have windings connected in series. Therefore, to implement a current control for such a system, all the lower half-bridge branches could be connected to ground through a sensing resistor, as shown in FIG. 3. By doing so, the total current, circulating in the motor windings could be controlled through the addition of the currents relative to the distinct windings. However, because of the generated back electromotive force, the current in the single winding will be modulated by the back electromotive force.
FIG. 4 shows the current through a motor winding for the type of current control as described above. With this type of control there is an inevitable loss of efficiency because the back electromotive force limits the current at the instants of maximum torque generation.
Another approach is that of controlling the current individually for each phase winding, and FIG. 5 shows the scheme of a brushless motor with independent windings where the driving is effected through three full-bridges, each of which drives a motor's winding. This requires, of course, a multiplication of the current control loops by the number of the windings.